The present invention generally relates to continuous wave (CW) Doppler ultrasound methods and apparatus utilizing a two-dimensional (2D) matrix array, and more particularly to CW Doppler methods and apparatus that utilize a dither signal component in the CW transmit signal.
Ultrasound systems have been proposed that afford a variety of techniques for examining an object of interest, whether it be human, animal, underground cavities, physical structures and the like. Examples of medical diagnostic imaging techniques include B-mode imaging, color flow imaging, power Doppler imaging, pulse-wave Doppler imaging, tissue velocity imaging, continuous wave Doppler imaging and the like. The various imaging techniques may include presenting to the user 2D or 3D images, temporal graphs (e.g., motion over time), and the like. During CW Doppler imaging, conventional systems drive an ultrasound probe to transmit a non-interrupted continuous wave ultrasound signal at a desired constant frequency. The waveform of the CW signal may be sinusoidal, a square wave and the like. In the frequency domain, the CW signal includes a single frequency component such that the associated frequency spectrum would exhibit a single spectral line proximate the carrier frequency (e.g., the transmitted CW signal would have substantially 0 bandwidth).
Conventional ultrasound systems have utilized a variety of probe constructions to carry out the associated examination technique. Examples of probes include transesophageal probes, convex probes, sector probes, bi-plane, tri-plane, and the like. The probe typically includes a group of transducer elements arranged in a one or two dimensional array. The transducer elements may be electronically controlled individually or in groups. Heretofore, ultrasound systems have utilized probes comprising a 1D array of elements to perform steerable CW Doppler imaging. The 1D array is divided into a first group of transducer elements proximate one section of the probe that continuously transmit the CW Doppler transmit signal and a second separate group of transducer elements that continuously receive echo signals.
In the probe, each transducer element is connected to one or more preamplifiers. Ultrasound systems that utilize 2D matrix probes have been proposed that perform sub-array (a.k.a. sub-aperture) beamforming in the probe to reduce the channel count connected to from the system from approximately 2000 to approximately 128 channels. The signals from a group (e.g., 4×4) of neighbor elements are delayed and summed in a sub-array processor, and the analog output from one subaperture processor is fed back to an associated channel of the receive (Rx) beamformer. The electronics to perform sub-array beamforming in the probe locates a large number of preamplifiers in the probe. Given the number of preamplifiers, it is desirable to use low-power preamplifiers in the probe.
Most ultrasound imaging modes (B-mode, Color Doppler, PW Doppler etc) transmit pulsed ultrasound. In pulsed ultrasound, the echo signals have different intensities when detected by the transducer elements depending upon the distance from the probe to the originating point of the echo signal. Echo signals originating deep within the object (far field) have lower intensity, while echo signals originating near the probe (near field) have higher intensity. In contrast, CW Doppler ultrasound is transmitted and received continuously, and thus signals from the near field and the far field are simultaneously present in the received signal thereby requiring a large dynamic range of the CW processing chain.
The receive preamplifiers in the probe have a limited dynamic range. The preamplifier dynamic range is closely correlated to the amplifier power requirements. As the number of transducer elements is increased, similarly the number of receive amplifiers is increased. As the number of amplifiers increases, power requirements also increase. In CW ultrasound mode, the gain of the preamplifiers is set so that their far-field sensitivity is maintained at a desired level. Due to dynamic range limitations, the receive preamplifiers may be driven into saturation by the near field echoes (sometimes referred to as carrier feed-through or cross-talk) a large fraction of the time. Driving the preamplifiers into saturation introduces errors in the CW signal processing chain.
A need remains for an ultrasound method and apparatus capable of reducing the effects of errors introduced into the processing chain. A need also remains for an ultrasound method and apparatus utilizing a low power probe having a 2D matrix array to perform CW ultrasound imaging.